Fakes have long been a plague of the art world. Thomas Hoving, the late director of the Metropolitan Museum of Art, estimated that he had examined some 50,000 pieces of art in his day, and that “fully 40 percent were either phonies or so hypocritically restored or so misattributed that they were just the same as forgeries.”

In Hoving’s view, “phonies” come in several varieties. “There’s the faker who knowingly makes the fraud; and the person who finds some work and deliberately claims it to be something far more exalted and the person who buys a work, learns it’s fake, and passes it off as genuine anyway.”

Jonathon Keats, a prominent art critic, has now stepped up to offer words in defense of “fakes.” In Roman times, he points out, craftsmanship was esteemed over authenticity, and a truly fine copy might even be signed by the person who copied it. In Mr. Keats‘ view, “No authentic modern masterpiece is as provocative as a great forgery.” He denounces Hoving’s preoccupation with forgery as a felony as “simplistic.”

Take religious icons. To medieval Christians, the authenticity of important icons was less important than their ability to perform miracles. One that performed well — consider the Shroud of Turin — was authentic, whatever its provenance. As for art, “the perfect copy was the perfect means to show off God-given talent.”

The profit motive, however, soon intruded. Roman sculptors found markets for copies of Greek statues. By the 19th century, forgery was big business, and it was to be found in all forms of art.

Perhaps the most versatile forger of the early-20th century was an Italian sculptor, Alceo Dossena. In the years after World War I, Dossena teamed with an unscrupulous art dealer, Alfredo Fasoli, to market skillful copies of Greek, Roman and Renaissance sculpture. Museums and collectors alike bought them as authentic antiques.

The collaborators’ greatest coup may have been the forging and marketing of a complete marble sarcophagus said to have been sculpted by the 15th-century master Mino da Fiesole. Although some experts expressed doubts as to its authenticity, Boston’s Museum of Fine Arts purchased it in 1924 for $100,000. All might have gone well had Dossena not had a falling out with his dealers and claimed credit for his many forgeries, including the Fiesole sarcophagus.

The Museum of Fine Arts at first refused to acknowledge that it had been had, but the tomb was removed from public view for a time. Eventually, it was returned to favor on the ground that whatever its provenance, “the tomb is put on exhibition because it is a beautiful object.” Presumably, it meets Mr. Keats‘ standard of “a great forgery.”

Far less skilled than Dossena yet equally disruptive to the art world was a Dutchman, Han Van Meegeren, who made a career of forging works of the great 17th-century artist, Jan Vermeer. The art world had long speculated that there were “lost” Vermeers to be discovered — products of a religious period when the artist was believed to have focused on biblical subjects. So the art world was receptive to the appearance of new Vermeers.

Van Meegeren the forger became rich in the years preceding and during World War II. But the fact that he dealt with the Nazis and their middlemen caused him to be tried after the war as a collaborator. Among other things, he was demonstrated to have sold a Vermeer, “Christ and the Adulteress,” to Herman Goering.

To save himself, Van Meegeren confessed that he was a forger, but insisted that he was a patriot who had deceived the avaricious Goering. To demonstrate his credentials, he dashed off a “Vermeer” for stunned Dutch authorities. His trial became a sensation, and Van Meegeren was ultimately acquitted of collaboration but sentenced to a year in prison for forgery.

The author is at his best in recounting these tales of notorious forgers, although the reader is conscious of the absence of illustrations. Mr. Keats is less persuasive when he philosophizes that copies are great art. By definition, copies lack spontaneity. More importantly, they lack any creative spark. Many artists might have copied the Sistine ceiling; only Michelangelo conceived it.